The accessories in a conventional motor vehicle are powered by output from the engine. An electric hybrid vehicle generally includes both an internal combustion engine and one or more electric motor/generators. Some of the accessories in a hybrid vehicle may require power while the engine is off and the vehicle is being powered by the electric motor/generator. Traditionally, in order to power hybrid vehicle accessories when the engine is off, it was necessary to provide each such accessory with a separate electric motor.
The typical automotive accessory drive system consists of a drive pulley connected to an output shaft of the engine, typically the crankshaft. Wrapped around this pulley is a flexible drive belt, which in turn is wrapped around a plurality of driven pulleys. This flexible drive belt transmits drive forces between the drive pulley and the driven pulleys. The driven pulleys may be fixably attached to accessories known in the art such as a power steering pump, air conditioning compressor, alternator, and water pump.
A belt-alternator-starter (BAS) system employs a combined starter and alternator motor/generator mounted with respect to the other components of the accessory drive system. This system combines the alternator and starter motor/generator into one device that can be mounted in effectively the same way and in effectively the same packaging space as a traditional alternator.